If it was closer to land, I would have suggested radio reception minimum altitude as the reason why, but recognizing todays navigation systems I guess navaids won't be recieved cross atlantic anyways.
Could it be the altitude floor of any airspace these routes pass through?
Or mabye because it is an upper airway, with upper airspace starting at FL245 some places... FL245 as the floor of upper airspace would give FL250 as the lower IFR flight level...